It is apparent that a resistance to corrosion resulting from molten metals is in great demand with respect to immersion members which are used over a long period of time in an immersed state in high temperature molten metal baths such as one of molten zinc, molten aluminum, or molten tin, or the like. In particular, in sink rolls and support rolls, it has been desirable not merely that resistance to corrosion resulting from molten metals be present, but also that abrasion resulting from the contact between the roll and the substrate to be plated, such as a steel plate or the like, which is immersed in the bath, be unlikely to occur, and that adhesion of metals also be unlikely to occur.
When metal adhesion occurs on immersion rolls such as sink rolls, support rolls or the like, damage is caused to the substrate to be plated, or to the plating surface of the steel plate or the like, which is guided by these rolls and immersed in the bath. Furthermore, for this reason, immersion rolls such as sink rolls and support rolls have become unsuitable for use.
Conventionally, in response to these varying demands, immersion members having various cermet materials thermal sprayed thereon have been developed and used; however, such members are as yet insufficient. For example, a WC-Co cermet thermal sprayed coating is used as an immersion member for use in molten metal baths; however, such a member is insufficient from the point of view of molten metal corrosion resistance.
Furthermore, the above-described demands have become more and more increasing in concert with demands for increasing quality of plated products, demands for a reduction in manufacturing costs, and demands for extended service life of immersion rolls.
In response to these demands, the present inventors previously invented an immersion member for use in molten zinc baths and the like, in which the surface coating of the immersion member itself comprises one or more of tungsten carbides, tungsten borides, and molybdenum borides, in addition to Co, and this was disclosed in Japanese Patent Application Hei 1-231293 (Japanese Patent Application, Laid-Open No. Hei 3-94048, laid open date: Apr. 18, 1991). Corrosion resistance of the immersion member with respect to molten metal baths was achieved by means of this invention; however, there was a problem in that corrosive peeling occurred during use over a long period of time.
In general, cracks and micropores are present in a thermal sprayed coating. During use of an immersion member in a molten metal bath over a long period of time, the molten metal penetrates to the interior of the thermal sprayed layer through these cracks and micropores and breaks down the thermal sprayed coating, corroding this thermal sprayed coating from below the surface, so that a phenomenon is noted in which the thermal sprayed coating peels away. This is termed corrosive peeling.
In order to solve this problem, the present inventors tested immersion members in which the cracks and micropores present in the thermal sprayed coating are filled with coal tar; however, under the conditions of high temperature present in the molten metal baths, the organic substances present in the coal tar decomposed and became gassified, and for this reason, the quality of the thermal sprayed coating was deteriorated, so that an immersion member having a long service life could not be obtained. Furthermore, the gas produced by the decomposition of the organic substances in the molten metal bath produced undesirable effects.
Furthermore, in order to avoid this phenomenon, an attempt was made to subject the immersion member to heat processing immediately prior to use in the molten metal bath after filling the cracks and micropores of the thermal sprayed coating of the immersion member for use in molten metal baths with coal tar; however, gas was produced by the decomposition of the organic substances contained in the coal tar during heat processing, and for this reason, micropitting was produced, and the coal tar filling material itself was lost, so that the desirable properties could not be obtained.